Electroactive polymers, methods of manufacture, and structures formed thereof

ABSTRACT

Methods for producing layered structures that include a conductive polymeric layer and a dielectric polymeric layer. The dielectric polymeric layer can be formed by curing a first volume of a dielectric polymeric material. A second volume of the dielectric polymeric material is doped with conductive particulates to yield a conductive polymeric material, which is then partially cured and solvated to create a conductive polymeric paste. The paste is applied to a surface of the dielectric polymeric layer, dried, and cured to form a conductive polymeric layer on the pre-strained dielectric polymeric layer yielding a layered structure that includes the conductive polymeric layer and the dielectric polymeric layer. A pre-strain is induced in the dielectric polymeric material by contacting a chemical thereto that causes swelling therein.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division patent application of co-pending U.S. patentapplication Ser. No. 15/556,696, filed Sep. 8, 2017, which claimspriority to International Patent Application No. PCT/US16/21778, filedMar. 10, 2016, which claims the benefit of U.S. Provisional ApplicationNo. 62/132,556, filed Mar. 13, 2015. The contents of these priorapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to polymeric structures. Theinvention particularly relates to methods for producing polymermaterials having electroactive properties and structures formed thereof.

A group of polymeric materials known as electroactive polymers (EAP)have been considered for various applications due to their ability toconvert electrical energy into mechanical motion through a process ofdeformation. Notable examples include actuators in which motion can beinduced by the application of electrical energy to an EAP material. EAPscan be divided into two subcategories, ionic and electric EAPs.Dielectric EAP materials are a subclass of electric EAPs that areviscoelastic and exhibit properties similar to dielectric materials ofcapacitors when positioned between two conductive electrodes. When asufficient electrical potential is applied to the electrodes, Coulombforces cause electrostatic stresses to occur that cause the viscoelasticEAP material to reallocate its volume, forcing it to constrict inthickness and expand (strain) in the in-plane (length and width)directions. This deformation brings the oppositely charged electrodesinto closer proximity relative to one another. When the electromagneticfield is removed, the EAP material substantially returns to its originalstate.

Electroactive polymer materials, particularly dielectric elastomers,exhibit improved performance in the form of increased deformation whenexposed to an electric field if the materials are pre-strained prior toimplementation of an electric field. In most applications, pre-strain isapplied using a mechanical stretcher and the polymer is retained on thestretcher.

U.S. Patent Application Publication No. 2015/0091254 discloses actuatorsand methods utilizing electrical properties of polymer materials,including but not limited to sealing systems, elements and methods. Suchactuators may comprise a multilayer structure that includes electrodesformed of electrically-conductive polymer materials, and anelectroactive polymer layer therebetween formed of a dielectricelastomer. The electroactive polymer layer is bonded to the electrodesso as to have a thickness dimension therebetween, and an electricpotential applied to the electrodes causes the electroactive polymerlayer to expand.

U.S. Patent Application Publication No. 2015/0091254 also disclosesmethods of fabricating such actuators by inducing a strain memory statein the electroactive polymer material using a pre-straining techniquethat expands the electroactive polymer material and then releases theelectroactive polymer material to allow the electroactive polymermaterial to substantially shrink to its pre-strained dimensions. Incontrast to the aforementioned mechanical pre-straining methods, aparticular example is a chemical pre-straining technique that isbelieved to be particularly suitable for certain EAP materials,including a fluorocarbon-based FKM EAP material.

There is an ongoing desire for improved methods of producing EAPmaterials and actuators formed therefrom.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides methods for pre-straining polymermaterials having electroactive properties, and producing layeredstructures from such materials, a nonlimiting example of which includesactuators.

According to one aspect of the invention, a method includes providing acured dielectric polymeric layer formed of a first volume of adielectric polymeric material, doping a second volume of the dielectricpolymeric material with conductive particulates to yield a conductivepolymeric material, partially curing the conductive polymeric material,solvating the conductive polymeric material to create a conductivepolymeric paste, applying the conductive polymeric paste to a surface ofthe dielectric polymeric layer wherein the conductive polymeric pasteincludes a chemical that causes the dielectric polymeric layer to swell,drying the conductive polymeric paste to form a conductive polymericlayer on the dielectric polymeric layer, allow the dielectric polymericmaterial to shrink, induce a pre-strain in the dielectric polymericlayer, and yield a layered structure comprising the conductive polymericlayer and the dielectric polymeric layer, and then curing the conductivepolymeric layer to bond the conductive polymeric layer to the dielectricpolymeric layer.

According to another aspect of the invention, a method includeschemically pre-straining a cured dielectric polymeric layer formed of afirst volume of a dielectric polymeric material by contacting thedielectric polymeric layer with a chemical that causes the dielectricpolymeric layer to swell, and then removing the dielectric polymericlayer from contact with the chemical to allow the dielectric polymericlayer to shrink, induce a pre-strain in the dielectric polymeric layer,and yield a pre-strained dielectric polymeric layer. The method furtherincludes doping a second volume of the dielectric polymeric materialwith conductive particulates to yield a conductive polymeric material,partially curing the conductive polymeric material, solvating theconductive polymeric material to create a conductive polymeric paste,applying the conductive polymeric paste to a surface of the pre-straineddielectric polymeric layer, drying the conductive polymeric paste toform a conductive polymeric layer on the pre-strained dielectricpolymeric layer and yield a layered structure comprising the conductivepolymeric layer and the pre-strained dielectric polymeric layer, andthen curing the conductive polymeric layer to bond the conductivepolymeric layer to the dielectric polymeric layer.

Technical effects of the methods described above preferably include theability to provide, manufacture, and use electroactive polymer actuatorsentirely composed of polymer-based materials for their conductive anddielectric components. The use of the same polymeric base material forthe dielectric and conductive components allows for the dielectric andconductive components to be cross-linked together, ensuring betterelectrical contact therebetween. In addition, use of the same polymerbase material reduces inhibition of deformation by the conductivecomponents.

Other aspects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are plan and cross-sectional views, respectively, thatschematically represent a planar multilayer configuration suitable foruse as an actuator in accordance with certain embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods for pre-straining polymermaterials having electroactive properties, and producing layeredstructures from such materials. In particular, the methods may be usedto manufacture actuators whose movement is controlled by the applicationof an electrical field to an electroactive polymers (EAP) material.Conversely, it is possible to generate an electrical field with anactuator containing an EAP material by physically actuating theactuator. One aspect of the invention is a construction of an actuatorusing only EAP materials.

Preferred EAP materials for use with the present invention include, butare not limited to, dielectric elastomers whose strain is nominallyproportional to the square of the activating electric field. Variousdielectric elastomers are known and can be used with the presentinvention, nonlimiting examples of which include VHB 4905 and 4910acrylic-based materials commercially available from 3M. Other notableEAP materials include fluoropolymers, particularly FKM(hexafluoropropylene vinylidene fluoride copolymer), commercial sourcesof which include Parker Seals, Inc.

FIGS. 1 and 2 schematically represent a nonlimiting actuator 10configured in accordance with a nonlimiting embodiment of the invention.As represented in FIGS. 1 and 2, the actuator 10 has a layered(multilayer) structure 12 comprising a layer 14 of an EAP materialdisposed between a pair of electrodes 16 and 18. An optional fourthlayer 20 is represented in FIGS. 1 and 2 that, if present, is preferablyalso formed of an EAP material. In the configuration represented inFIGS. 1 and 2, the EAP material layer 14 between the electrodes 16 and18 is referred to as an active layer because it is subjected to anelectrical field applied as a result of an electrical potential beingapplied across the electrodes 16 and 18. When an electromagnetic fieldis applied through the electrodes 16 and 18, the EAP material of thelayer 14 reallocates its volume, compressing in the thickness directionand expanding on the plane transverse to the thickness direction. Thefourth layer 20 can be considered to be an inactive layer of theactuator 10, and its primary role is to prevent shorting between theelectrode 16 and surrounding components of a system in which theactuator 10 is installed. Alternatively, the fourth layer 20 may also bean active layer and the circuit may be repeated, that is, multipleactuators 10 may be stacked in order to increase the force and actuationpotential. The layers 14, 16, 18 and 20 contact and are bonded to eachother so that the layers 14, 16, 18 and 20 expand and contract largelyin unison, primarily in the plane parallel to the layers 14, 16, 18 and20. For this purpose, adhesives or compression molding techniques may beused, though in preferred embodiments the electrodes 16 and 18 areapplied to the EAP layer 14 by roll to roll processes, additivemanufacturing, or screen printing processes and the layers 14, 16, 18and 20 are bonded together by co-curing. Various co-curing techniquesare foreseeable and within the scope of the invention. In the embodimentrepresented in FIGS. 1 and 2, the electrodes 16 and 18 are contacted bymetal leads 22 through which the electrical potential can be applied tothe electrodes 16 and 18. A wide variety of flexible and substantiallyrigid conductive materials can be used to form the leads 22.

As in U.S. Patent Application Publication No. 2015/0091254, methods forproducing the multilayer structure 12 utilize a chemical pre-strainingtechnique to produce the EAP layer 14 that does not require the strainto be retained in its EAP material. Such a chemical treatment processgenerally entails contacting a sheet or film of a cured EAP materialwith a chemical that causes the cured EAP material to swell. Thetreatment is carried out until the EAP material has sufficientlyswelled, as a nonlimiting example, as evidenced by a linear dimensionalincrease of about 100 percent or more. Thereafter, the EAP material isremoved from contact with the chemical and allowed to dry, resulting inthe EAP material substantially shrinking back to its originaldimensions, for example, within about 5% of its original dimensions.

In a particular but nonlimiting example, the EAP material can be afluoropolymer (FKM), and the chemical pre-straining technique uses amethyl ethyl ketone (MEK), which is known to cause cured FKM to swelldue to an amine reaction. This reaction causes intercalation of thesolutes into the polymer matrix of FKM. In one investigation, a volumeof FKM was cured to form a layer of cured FKM that was then placed inMEK for about five minutes, resulting in the FKM at least doubling involume. If the FKM was not cured prior to contact with MEK, the MEKwould dissolve the FKM. The FKM was then allowed to dry for at leasttwelve hours, which allowed the FKM to return to roughly its originalsize. The swelling reaction was determined to be greater than 95%reversible. Furthermore, the MEK caused the volume of FKM to swellevenly in all directions, allowing for more uniform strain when comparedto mechanical stretching.

While not wishing to be held to any particular theory, with this processthe EAP layer 14 and, optionally, the EAP layer 20 appeared to retain astrain memory, allowing for the electrodes 16 and 18 to be attachedthereto while the layers 14 and 20 are not in the process of beingpre-strained or are in a physically pre-strained condition. Instead, theelectrodes 16 and 18 can be attached to the EAP layers 14 and 20 afterpre-strain has been released (i.e., their EAP materials are no longerswelled). Such a technique is in contrast to prior practices that entailintentionally retaining a pre-strain in an EAP material duringapplication of the electrodes, for example, with a stiffened regionsurrounding a pre-strained region of an EAP material to continuouslyapply a strain to the pre-strained region while the electrodes are beingattached. Consequently, the present invention encompasses a method offabricating an actuator that entails pre-straining an EAP material, andthen releasing the strain to induce a strain memory in the EAP materialprior to application of electrodes thereto, and until such a time as theresulting actuator (10) is activated by the application of an electricfield with the electrodes (16 and 18). Though investigations leading tothe invention induced strain memory in an EAP material through achemical treatment that caused the material to swell, it is foreseeablethat strain memory could be induced in a variety of EAP materialsthrough the use of other pre-straining techniques that expand the EAPmaterial and then release the EAP material to allow the material toshrink and return or nearly return to its pre-strained dimensions.Suitable techniques for pre-straining the EAP layers 14 and 20 includemechanical, electrical, radiation, and thermal techniques of types knownin the art. For example, pre-straining of the EAP layers 14 and 20 canbe mechanically induced with the use of unidirectional, bidirectional,and omnidirectional stretching equipment.

Unlike U.S. Patent Application Publication No. 2015/0091254, which usedelectrically-conductive polymer materials that exhibit flexibilitycomparable to the EAP layer 14, such as mixtures of conductive greasemixed with graphite, silver inks or paints, mixtures of silicone andgraphite, and electrically-conductive silicone-based rubber materials,preferred embodiments of the present invention form the electrodes 16and 18 from the same EAP material used for manufacturing the EAP layer14. In accordance with a nonlimiting embodiment of the invention, one orboth of the electrodes 16 and 18 may be produced by doping a volume of adielectric polymeric material having the same composition as that of theEAP layer 14 with conductive particulates to yield a conductive polymermaterial having the same base polymeric composition as that of the EAPlayer 14. The conductive polymer material is partially cured, whichenables the material to be at least partially solvated to create aconductive polymeric paste or paint. This paste may be applied toopposite sides of the cured EAP layer 14 (in which case the layer 14serves as a substrate) and then dried to form the layers 16 and 18 onthe pre-strained EAP layer 14. Application of the paste to the EAP layer14 may induce additional pre-strain in the EAP layer 14. If layer 20 isdesired, it may be attached at this point. Preferably, the layers 16 and18 are then bonded to the layers 14 and 20 by curing the layers 16 and18.

For example, the aforementioned investigations further involved theapplication of a conductive layer to a pre-strained dielectric FKM layerformed as described above. In this process, a second volume of FKM wasdoped with conductive particulates and partially cured. Thepartially-cured, doped FKM material was then solvated using MEK,creating a paste which was then painted onto a surface area of thepre-strained dielectric FKM layer. The paste was then allowed to dry toform a conductive layer of the partially-cured, doped FKM material. TheMEK in the conductive layer caused additional pre-strain in thepre-strained dielectric FKM layer, thereby improving deformationpotential. The resulting layered structure was then cured at about 425°F. The use of the same polymeric base material for the dielectric andconductive layers of the layered structure allowed for a more durabledesign because all layers (dielectric and conductive) were cross-linkedtogether, ensuring better electrical contact between the dielectric andconductive layers. In addition, use of the same polymeric base materialdrastically reduced inhibition of deformation by the conductive layer.

In the above processes, the cured EAP layer 14 is pre-strained by thechemical straining process prior to application of the conductivepolymeric paste thereto, which may cause additional pre-straining in theEAP layer 14. As an alternative, the conductive polymeric paste may beapplied to the cured EAP layer 14 prior to any pre-straining of the EAPlayer 14, and a chemical in the conductive polymeric paste (for example,MEK) may be utilized to cause swelling of the EAP layer 14 and createpre-straining therein. In yet another alternative method, conductivepolymeric layers and dielectric polymeric layers may be contacted andco-cured to form the layers 14, 16, and 18 (optionally layer 20), andthen the stack may be contacted with a chemical to cause swelling andpre-strain in the EAP layer 14.

In view of the above, electroactive polymer actuators can be fabricatedwhose conductive and dielectric components are entirely composed ofpolymer-based materials, and preferably the very same polymeric basematerials. Actuators can be fabricated by curing a layered structurecomprising solvated conductive layers applied to opposite surfaces of apre-strained dielectric layer as described above. Such a chemicalpre-strain and electrode application method can be used for manymaterials other than FKM. As nonlimiting examples, ethylene propylenerubber (EPDM) and silicone swell in petroleum oil and fuel, nitriles(for example, nitrile rubber (NBR)) swell in toluene, and VHB acrylics(3M) swell in solvents, and therefore are candidates for the chemicalpre-straining technique described above. Preferred polymeric materialsfor a given application will depend on desired material properties of alayered structure.

Applications for layered structures as described above include, but arenot limited to, seals having the ability to reseal a leak, and actuatorsfor use in such varied applications as automotive systems (for example,to alert drivers to hazards) and medical applications (for example, topromote circulation, control urinary incontinence, pumps, etc.).

While the invention has been described in terms of specific embodiments,it is apparent that other forms could be adopted by one skilled in theart. For example, the physical configuration of the actuator 10 coulddiffer from that shown, and materials and processes/methods other thanthose noted could be used. Therefore, the scope of the invention is tobe limited only by the following claims.

1. A method comprising: providing a cured dielectric polymeric layerformed of a first volume of a dielectric polymeric material; doping asecond volume of the dielectric polymeric material with conductiveparticulates to yield a conductive polymeric material; partially curingthe conductive polymeric material; at least partially solvating theconductive polymeric material to create a conductive polymeric paste;applying the conductive polymeric paste to a surface of the dielectricpolymeric layer, the conductive polymeric paste comprising a chemicalthat causes the dielectric polymeric layer to swell; drying theconductive polymeric paste to form a conductive polymeric layer on thedielectric polymeric layer, allow the dielectric polymeric material toshrink, induce a pre-strain in the dielectric polymeric layer, and yielda layered structure comprising the conductive polymeric layer and thedielectric polymeric layer; and then curing the conductive polymericlayer to bond the conductive polymeric layer to the dielectric polymericlayer.
 2. The method according to claim 1, further comprising:chemically pre-straining the cured dielectric polymeric layer bycontacting the dielectric polymeric material with the chemical thatcauses the dielectric polymeric layer to swell, and then removing thedielectric polymeric layer from contact with the chemical to allow thedielectric polymeric layer to shrink and induce an initial pre-strain inthe dielectric polymeric layer prior to applying the conductivepolymeric paste to the surface of the dielectric polymeric layer.
 3. Themethod according to claim 1, wherein the conductive polymeric layer is afirst conductive polymeric layer, and the method produces an actuatorcomprising the layered structure and a second conductive polymeric layerformed on the dielectric polymeric layer.
 4. The method according toclaim 3, wherein the second conductive polymeric layer is formed by thesame process as the first conductive polymeric layer.
 5. The methodaccording to claim 1, wherein the dielectric polymeric material is afluoropolymer, and the chemical is methyl ethyl ketone.
 6. The methodaccording to claim 1, wherein the dielectric polymeric material is anethylene propylene rubber or a silicone-based material, and the chemicalis a petroleum-based oil or fuel.
 7. The method according to claim 1,wherein the dielectric polymeric material is a nitrile material, and thechemical is toluene.
 8. A method comprising: chemically pre-straining acured dielectric polymeric layer formed of a first volume of adielectric polymeric material by contacting the cured dielectricpolymeric layer with a chemical that causes the cured dielectricpolymeric layer to swell, and then removing the cured dielectricpolymeric layer from contact with the chemical to allow the cureddielectric polymeric layer to shrink, induce a pre-strain in the cureddielectric polymeric layer, and yield a pre-strained dielectricpolymeric layer; doping a second volume of the dielectric polymericmaterial with conductive particulates to yield a conductive polymericmaterial; partially curing the conductive polymeric material; at leastpartially solvating the conductive polymeric material to create aconductive polymeric paste; applying the conductive polymeric paste to asurface of the pre-strained dielectric polymeric layer; drying theconductive polymeric paste to form a conductive polymeric layer on thepre-strained dielectric polymeric layer, and yield a layered structurecomprising the conductive polymeric layer and the pre-straineddielectric polymeric layer; and then curing the conductive polymericlayer to bond the conductive polymeric layer to the pre-straineddielectric polymeric layer.
 9. The method according to claim 8, whereinthe conductive polymeric layer is a first conductive polymeric layer,and the method produces an actuator comprising the layered structure anda second conductive polymeric layer formed on the pre-straineddielectric polymeric layer.
 10. The method according to claim 8, whereinthe second conductive polymeric layer is formed by the same process asthe first conductive polymeric layer.
 11. The method according to claim8, wherein applying the conductive polymeric paste to the surface of thepre-strained dielectric polymeric layer, and drying the conductivepolymeric paste to form the conductive polymeric layer on thepre-strained dielectric polymeric layer induces additional pre-strain inthe pre-strained dielectric polymeric layer.
 12. The method according toclaim 8, wherein the dielectric polymeric material is a fluoropolymer,and the chemical is methyl ethyl ketone.
 13. The method according toclaim 8, wherein the dielectric polymeric material is an ethylenepropylene rubber or a silicone-based material, and the chemical is apetroleum-based oil or fuel.
 14. The method according to claim 8,wherein the dielectric polymeric material is a nitrile material, and thechemical is toluene.